Opto-isolators are frequently used as replacements for transformers in providing DC ground isolation in power circuits. Their low cost, low weight and ability to be enbodied in integrated circuit form are particularly advantageous in small board mounted circuits. A dual detector opto-isolator containing one light emitting device and two light detecting devices is often used when a negative feedback signal is needed to linearize the light emitting diode current-voltage characteristic independently of temperature. Since the light emitting device supplies the basic source emission for both the output and feedback signal, no additional feedback circuitry coupled to the output signal is necessary.
A problem encountered in such an application of this device is related to the fact that the transfer gain from the light emitting device to each of the light detecting devices differs from each other and from one opto-isolator to another. Hence in an amplification application requiring ground isolation between inputs and output, the variability of the current gains in both forward current and feedback current will affect operation of individual amplifier units and cause them to be different from one another. This is particularly true in situations requiring DC signal transmission. Prior arrangements concerned with the opto-isolator variability such as disclosed in U.S. Pat. No. 4,190,747 issued to A. Feiner et al have been concerned with controlling linearity of an AC signal transfer through a dual detector opto-isolator. Arrangements for properly utilizing a dual detector opto-isolator in high accuracy transmission of DC signals have not been priorily disclosed.
Opto-isolator arrangements having one emitting device and two detecting devices which are utilized to transmit DC signals must account for both DC contrast and the current transfer ratio. The current transfer ratio represents the ratio of the DC current from emitting device to the detector device and the DC contrast is the ratio of the current transfer ratios of the two detecting diodes.
These factors must be controllable and fully accounted for in any power circuit utilizing a power amplifier circuit to transmit sensed DC voltages across a ground isolation circuit including a dual detector opto-isolator to isolate the electrical ground of input and output.